Decoupled feedforward-feedback control system

ABSTRACT

A control system combining feedforward and feedback control of a manipulated variable of a process is arranged to provide a modification of the error signal to which the feedback control responds so as to effect a decoupling of the feedforward and feedback control actions. The error signal is diminished by a signal of magnitude sufficient to minimize or reduce by a predetermined amount the response by the feedback control to action by the feedforward control as reflected in changes in the controlled variable. Provision is also made to avoid unnecessary feedback control in response to set point changes when feedforward action is provided from the set point changes.

Littman et al.

DECOUPLED FEEDFORWARD-FEEDBACK CONTROL SYSTEM Inventors: BernardLittman, Fhiladelpmaf Charles W. Ross, Hatboro, both of Pa.

Assignee: Leeds & Northrup Company,

Philadelphia, Pa.

Filed: July 10, 1972 Appl. No.: 270,419

US. Cl. 235/150.l, 60/108/9, 318/561 Int. CL... G05b 13/02, GOSb 17/02,GOSd 7/06 Field of Search 235/150.1

Bristol 235/150.]

[4 1 Sept. 11, 1973 Primary ExaminerEugene G. Botz Attorney-William G.Miller, Jr. and Raymond F.

MacKay et a1 [57] ABSTRACT A control system combining feedforward andfeedback control of a manipulated variable of a process is arranged toprovide a modification of the error signal to which the feedback controlresponds so as to effect a decoupling of the feedforward and feedbackcontrol actions. The error signal is diminished by a signal of magnitudesufficient to minimize or reduce by a predetermined amount the responseby the feedback control to action by the feedforward control asreflected in changes in the controlled variable. Provision is also madeto avoid unnecessary feedback control in response to set point changeswhen feedforward action is provided from the set point changes.

21 Claims, 6 Drawing Figures 1 l 18 m PROCESS FEEDBACK MODEL eCONTROLLER PROCESS t a 20 22 0 26 26 12 GP 14 1s PAIENIE'D swim or 4 I Il I I l I I I I I I I I I I I I l I I I I I I I I I I J DECOUPLEDFEEDFORWARD-FEEDBACK, CONTROL SYSTEM BACKGROUND OF THE INVENTION In manycontrol systems it has been proven advantageous to utilize a feedforwardcontrol in addition to the conventional feedback control system for thepurpose of improving the overall control performance. For the purposesof this description, feedforward control action may be defined as thecontrol action in which information concerning either a controller setpoint or a demand signal is converted into corrective action on themanipulated variable to minimize deviations of the controlled variablefrom the set point. A demand signal may be defined as the representationof a process related variable which affects the controlled variable in apredictable manner. In applications involving a single feedback controlloop, for example, a feedforward control signal derived from a processcondition such as the set point signal may be used to provide improvedresponse to changes inset point. The feedforward signal wouldconventionally modify the manipulated variable of the control system byproviding proportional and/or derivative control response.

In control systems in which both feedforward and feedback control actionis required it has been found that the combination of those two controlactions may either lead to an excessive control action or requirecompromises in the tuning of the control system to prevent excessivecontrol action. It is normally desirable to tune the feedback controllerfor optimum response to disturbances in the system being controlled andat the same time it is desirable that the feedforward control action beindependently tuned for optimumresponse to changes in demand or setpoint. Tunings for optimum response for both the feedforwardand feedbackcontrol actions have, however, aspointed out, been found to be difficultto obtain because of the coupling between the feedforward and feedbackcontrol systems.

It is an object of this invention to provide a means for decoupling thefeedforward and feedback controlsystems so that each may be tuned foroptimum response.

SUMMARY OF THE INVENTION In carrying out this invention there isprovided a method for controlling a variable of a process to a set pointby automatically modifying by feedback control action a manipulatedvariable of the process toreduce the effective error between thecontrolled variable and the set point toward zero while automaticallymodifying by feedforward control action the manipulated variable of theprocess in accordance with the change of a process condition and toallow optimum tuning of the feedforward and feedback control action.There is utilized an automatic modification of the effective errorsignal so as to prevent a change in the effective error signal inresponse to the feedforward portion of the signal to which thefeedforward control action responds. In carrying out such a controlmethod where the control system has means for providing feedforward andfeedback control action this invention provides the improvement whichcomprises means for modifying the effective error signal so as toprevent a change in the effective error signal in response to changes inthat signal to which the feedforward control action responds.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings in which likereference characters identify like elements:

FIG. 1 is a block diagram of a feedforward-feedback control systeminvolving a single control loop.

FIG. 2 is a block diagram of a feedforward-feedback control systemshowing a modification of the system of FIG. 1.

FIG. 3 is a block diagram of a feedforward-feedback control systemshowing feedforward control action from the set point as shown in FIG. 2with the addition of feedforward control action from a separate demandsignal.

FIG. 4 is a block diagram of a feedforward-feedback control system asapplied to a boiler-turbine control in which the throttle pressure isallowed to vary while the throttle valve position is controlled to aconstant set point.

FIG. 5 is a circuit diagram in block form showing a form of processmodel for modeling a portion of a steam boiler.

FIG. 6 is a circuit diagram in block form showing a feedforwardcontroller providing both rate and proportional responses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a system forcontrolling the process identified as block 10 which has a transferfunction G p by modifying the manipulated variable m shown as a signalon the line 12 into the process 10 in order to maintain at a set pointthe controlled variable 0. The variable c may be measured to provide asignal on line 14 indicative of the value of that variable of theprocess. As in the normal feedback control loop the value of thecontrolled variable c is introduced by way of line 16 to subtractor 18where it is compared with a value which acts as a set point and whichappears in theform of a signal on line 20 as another input to thesubtractor 18. Thedeviation between the controlled variable appearing online 16 and the set point signal appearing on line 20 appears in theform of an error signal on output line 22from subtractor 18. That errorsignal is then the effective error signal'to which the feedbackcontroller 24 responds. Controller 24 is shown as having a transferfunction G The feedback controller may be any of a number of well knownfeedback controllers and may, for example, provide proportional,derivative and integral response to the error signal. Such response willchange the manipulated variable by modifying the signal m on line 26which will in turn modify the signal on line 12, since the signal m online 26 is introduced as an input to the summer 28 whose output on line12 is the signal m representing the manipulated variable. Forconvenience m,, m and m may be referred to as manipulated variables.

In FIG. 1 there is provided a set point signal r on line 30 which signalmay be modified to change the set point for the process 10. When the setpoint 30 is subject to changes it is desirable to provide feedforward bymeans of the apparatus of block 32. The block 32 is designed to utilizeas an input the signal r on line 34 as it is derived from line 30 toprovide on its output line 36 a signal relating to the input inaccordance with the Laplace transform which in effect provides on line36 a lagged derivative signal, or in other words, a lagged rate signalso that the feedforward control action will be proportional to the rateof change of the set point with that signal being lagged by apredetermined amount. The signals on lines 30 and 36 are summed at thesummer 38 to provide an input to block 40 where the results of thesummation are multiplied by the reciprocal of the gain of the processmodel 1/ IG I, the product appearing as a signal on output line 42 fromblock 40. The symbol Gpul is used to represent steady state gain of thetransfer function G The signal from line 42 is then introduced as afeedforward control action modifying the manipulated variable m byadding to the manipulated variable m appearing on line 26 as a signal, mwhich is determined by modifying the signal on line 42 by a constant Kwhich multiplication is carried out in block 44. Thus, the input toblock 44 from line 42 which appears on line 46 is multiplied by aproportional gain K in block 44 to provide on its output line 48 thesignal m,.

The signal from line 42 is also introduced as an input to process model50 which is shown as having a transfer function G which then providesits output on line 20. The process model 50 is desirably a circuit whichis designed to model the process 10 as closely as is practicable withthe output representing a total signal as compared with an increment aswould be obtained from a linearized model. The development of suchprocess models is an art well known and therefore needs no furtherexplanation.

ln decoupling the feedback and feedforward control actions of the systemin FIG. 1 it is desirable that there should not be any change in theerror signale in response to a change in the set point r. To accomplishthat decoupling, the effect on the controlled variable c of thefeedforward control action is canceled. Thus, as modification of thesignal m, changes the controlled variable c, that effect is canceled atthe subtractor 18 by the change in the signal on line 20. The signal online 20 is tailored by the process model 50 to respond to changes in thesignal on line 42, representing the feedforward control action, in thesame way as that feedforward control action affects the'controlledvariable c. The cancellation of the changes in the signal on line 16 dueto feedforward control action by changes in the signal on line 20assumes that the proportional gain K introduced by block 44 has a valueof one. When the value of K is less than one then instead of the changein signal on line 16 being canceled by a change in signal on line 20,the change on line 20 exceeds the change on line 16 creating a change inthe error on line 22 which is eventually reduced by the feedback controlaction. v

As is the usual case the changes in the controlled variable resultingfrom disturbances to the process are corrected by the operation of thefeedback controller to change the signal m on line 26 until theresulting change in m causes the signal 0 on line 16 to come to equalitywith the signal on line 20.

It will be evident that in the steady state condition the signal on line20 will be r and hence the process will be controlled to cause c toequal r.

In FIG. 2 there is shown another system for providing a combination offeedforward and feedback control action with the novel decouplingsystem. This system differs in several respects from the system of FIG.1, particularly in that the coefficient [:2 is introduced. As shown inFIG. 1 the process 10 has its manipulated variable modified by thesignal m on line 12 so as to maintain through the operation of thefeedback controller 24 a value for the controlled variable ccorresponding with the set point signal r. As shown in FIG. 2, thecontrolled variable c is introduced by way of line 16 to a subtractor 60which compares the set point r and and the controlled variable c andprovides an output signal on line 62 representative of the error e, orin other words, the deviation of the controlled variable from the setpoint. The error signal e is then modified by subtractor 64 which has asone of its inputs the signal from line 62 while the other input is asignal from line 66. The comparison of the signals from lines 62 and 66provide an effective error signal e* which appears on line 68 and is theeffective error signal to which the feedback controller 24 responds.

The feedback controller 24 then produces a signal representing themanipulated variable m on line 26 which signal is added to the signalfrom line 48 in the adder 28 in a manner similar to that shown inFIG. 1. As in FIG. 1, the signal from line 48 provides the feedforwardcontrol action which is derived by the use of a feedforward controller70 shown in FIG. 2 as including the feedforward dynamics of block 32similar to that shown in FIG. 1 which provide on an output line 36 alagged rate signal responsive to changes in set point r by way of theintroduction of that set point signal as an input on line 34 to block32. As in FIG. 1, the signal on line 36 is summed with the set point rat the summer 38 and the output of that summer is then introduced as aninput to multiplier 72 which multiplies the output of the summer by thefactor K representing the proportional gain of the feedforward controlaction. The output of the block 72 on line 74 is then the feedforwardcontrol action which is modified by multiplying it by a factorrepresenting the reciprocal of the gain of the process model as shown inblock 76. The result of that multiplication produces the signal m, whichrepresents the feedforward control action applied to the process 10 byway of the summer 28 and line 12.

In FIG. 2 in order to decouple the feedback control action and thefeedforward control action so that they may be separately tuned and sothat there may be an avoidance of excessive control as is sometimesexperienced in using both the feedforward and feedback control there isprovided a non-linear process model 50 which has as its input the signalm, from line and which produces on its output line 82 a signalrepresenting the anticipated value for the controlled variable 0 whichshould result from the effect of the feedforward control action m, onthe process 10 so that the signal on line 82, by way of the circuitrylater to be described, is effective to cancel or at least diminish by apredetermined amount the effect of the change in the signal appearing online 16 as a result of the feedforward control action so that theeffective error signal e will not be affected by the feedforward controlaction or will be affected only to a diminished degree.

When the control system is in its steady state the set point signal rwill be canceled by the signal c on line 16 so that the error signal eproduced by subtractor 60 is zero. Under steady state conditions thesignal on line 66 will be zero so that the effective error signal e* online 68 is also zero. The value of the signal on line 66, when there isa change in the set point r, is obtained by multiplying the signal r bya gain factor K in the multiplier 88. The signal from the output of themultiplier 88, on line 90, is transmitted through subtractor 92 and themultiplier 86, where it is multiplied by B, to line 66 so that it isintroduced into subtractor 64 to cancel out or diminish the effect ofany change of the signal r on the signal c. It will be evident that ifthe factors K in blocks 88 and 72 are equal to one and if the value of[3 in block 86 is equal to one, the operation of the circuitry of FIG. 2will be similar to that of FIG. 1 with K in block 44 equal to one.

Under some conditions it is desirable to use less than the fullfeedforward control action. For example, it may be desirable that theproportional gain of the feedforward control K have a value of 0.7. Whenthat is the case the changes which occur in the set point r will not becompletely canceled by the signal on line 66. Instead, there will beintroduced into the feedback controller 24 an effective error signal e*such as to cause the feedback controller to respond to three tenths ofthe change in the set point while the feedforward control actionprovides the response to the seven tenths.

As changes occur in the value of the controlled variable c due to thefeedforward control action, those changes will introduce a variationinto the error signal e on line 62 and it is necessary for the operationof this invention that those changes be canceled out by the signal online 66. This is accomplished by the effect on the signal on line 66 ofthe output of the process model 50 in response to the feedforwardcontrol action represented by changes in the signal m;. The signal online 82 will vary as does c and thus if B is one, the change in thesignal on line 82 will result in a canceling of the change in the signalon line 16 due to feedforward control action. In the steady statesituation the signal on line 82 will cancel the signal on line 90 insubtractor 92, thus the value of the signal on line 66 would be zero.Under those conditions the feedback controller 24 will maintain thecontrolled variable c at a value equal to the set point r.

lfB is less than one, then only a portion of the signal for cancelingthe effect on e appears on line 66 so that the decoupling of thefeedforward and feedback control is diminished. If, for example, thevalue ofB is set to zero then the benefits of this invention are notbeing used.

It will be evident to those skilled in the art that the feedforwardcontroller may provide a proportional action only, a rate action only orany desired combination of those control responses. Similarly, theprocess model may he a simplified model of the process such as will beadequate to produce the necessary degree of decoupling between thefeedforward and feedback control actions.

The present invention is applicable to control systems in which a setpoint change and a change in the demand on the process may both occur.Such a system is shown in FIG. 3, wherein the demand change may bedetected by the measurement of a system variable indicative of thedemand on the process. In FIG. 3 the process may, for example, be aboiler-turbine system with the manipulated variable m being the firingrate of the boiler and the other manipulated variable of the process m',the position of the throttle valve adjusting the flow of steam from theboiler to the turbine. The control of the manipulated variable m may befrom another control system which may be controlling the output of thegenerator driven by the turbine. The value of m is indicative of thedemand on 'the process. Thus, if e is a measure of boiler pressure itwill be evident that m can be considered a demand signal for the controlsystem modifying m since a change in steam flow due to a change inthrottle valve opening will cause a pressure change which musteventually be compensated for by modifying the firing rate of theboiler.

In FIG. 3 the control system for modifying m is similar to that shown inFIG. 2 with the addition of a feedforward control of the manipulatedvariable m from the demand signal. To provide such a feedforward controlaction there is provided feedforward controllerlllt), having a transferfunction G for responding to the demand signal m. on line 102 to providean output on line 104 to summer 106 where it is summed with thefeedforward control signal produced by feedforward controller asmodified by multiplier 76. The sum of the two feedforward signalsproduces the signal m, on line 108 for summing with the signal m insummer 28, as in FIG. 2. The signal m, also appears on line 110 as oneof the inputs to the process model 112, which is shown as having atransfer function G The other input to the process model 112 is by wayof line 114 from line 102 and represents m, the demand signal. Thesignal from line 114 serves to modify the process model 112 in the sameway that the signal m modifies the process 95 itself. Thus, the processmodel 112 will model in non-linear fashion the relationship between mand c of process 95 as that relationship is modified by the effect of m.

In FIG. 3 if the set point is a fixed value, then the feedforwardcontroller 70 and the multiplier 76 are not necessary. Also, line 90 mayalso be omitted if a linearized model is used instead of the non-linearmodel 1 12.

In FIG. 4 the process is shown as including a steam boiler 182 whosefuel supply is provided by way of line 184 through valve 186. Valve 186is under the control of valve positioner 188 so that the valve 186 ispositioned in proportion to the magnitude of the input signal topositioner 188, namely, the magnitude of m on line 12. As shown, theboiler 182 produces steam on its output line 190 through throttle valve192 to turbine 194. The turbine 194 in turn operates to drive thegenerator 196. In the process 180 the controlled variable P representingthe throttle pressure, is measured from steam line 190 to provide thesignal on line 16. There is also a measurement made of the first stagepressure in turbine 194 by the pressure measuring device 198 to providean indication of the first stage pressure, namely, P

By dividing the value of P by the value of P in the divider 200, theratio P /P is produced on line 202 as an indication of the position ofvalve 192. The process 180 including boiler 182, may, for example,operate in a sliding pressure mode such as is described in US. Pat.

application, Ser. No. 2l7,484, filed Jan. 13, 1972, by our coworker T.W. Jenkins, Jr. As explained in that application, a single indication ofthe process demand may be obtained by multiplying the ratio P /P by thethrottle pressure set point P It will be noted from the drawing of FIG.4 that the set point is provided on line 30 and the product referred tois provided by the multiplier 210 so that the demand signal then appearson line 212 as a signal to feedforward controller 70. As further shownin FIG. 4 the output of the feedforward controller 70 is utilized inmuch the same way as in FIG. 3 with the output of the controller 70being multiplied by the reciprocal of the gain of the process model withthe resulting product providing one of the inputs to process model 113,namely, the input on line 110.

In FIG. 4 it is necessary to take into account the fact that the setpoint P will be changing as indicated in the description of the variablepressure control system of the above mentioned Jenkins application.There is thus provided to line 90 a signal which will tend to cancel outthe effect on the error e of the set point changes so that the effectiveerror e* will remain essentially unchanged as a result of changes in setpoint provided that the factor B in the multiplier 86 is set to a valueof l.

The signal on line 202, which provides an input through line 1 14 toprocess model 113 is effective as its value changes to modify the outputof the linearized process model on line 82 in addition to the effect onthe output caused by the signal on line 110. The signal on line 82represents a change in the value of the controlled variable P and cannotnormally be canceled by the signal on line 90 and thus it is necessaryto introduce into the summer 214 a signal (P J indicative of the initialcondition for the value of the throttle pressure set point which is thevalue for the load for which the model 113 is designed.

It will thus be evident from FIG. 4 that a boilerturbine system may becontrolled in a sliding pressure mode utilizing feedforward control froma process demand signal in such a way that there is no interactionbetween the feedforward control and the feedback control when thecontrol system is designed in accordance with the principles establishedherein and exemplified by the systems of FIGS. 1-3. Thus, thearrangement of FIG. 4 operates in a manner similar to that of FIG. 3,for example, with the exception that an external demand signal 202 ismultiplied by the set point signal on line 33 to obtain a compositedemand signal on line 212. In addition, the arrangement of FIG. 4 makesthe assumption that the proportional gain K is equal to 1 thus making itpossible to omit block 88. Likewise, since the demand signal isestablished by multiplying the signal on line 202 by the set point fromline 30 it is only necessary to have one feedforward controller 70 andhence the controller I00 of FIG. 3 can be omitted.

The arrangement of FIG. 4 for the control ofa boilerturbine system in asliding pressure mode can be readily adapted by those skilled in the artto control a boilerturbine system operated with a constant throttlepressure. The necessary modifications would include the omission of theline 90 as well as the line 216, thus making the summer 214 unnecessary.The line 33 could also be omitted along with multiplier 210 so that thesignal on line 202 would provide the demand signal input through thefeedforward controller 70.

In boiler control systems, the value (P,/P P may as mentioned be used asa demand signal in a boiler control system. Likewise, P,/P desiredgeneration or steam flow and other similar variables may be used asdemand signals.

The process model 113 may be a linearized model of a portion of a steamboiler, for example, and may utilize a circuit such as that shown inFIG. 5, which models the boiler firing dynamics and the boiler pressuredynamics. The model is essentially a series arrangement of two firstorder lags. The first of the two lags uses integrator 120 and the inputfrom line 110. The input signal from line first is adjusted bypotentiometer 124 to give an appropriate input to the integrator whichhas a potentiometer in its feedback path, namely, potentiometer 126,thus forming the initial first order lag. The output of the initial lagon line is used as an input through potentiometer 131 to the second lag,made up of integrator 132 and the potentiometer 134 in its feedbackcircuit. Integrator 132 also has a second input from line 114 throughpotentiometer 136. The output of integrator 132 on line is then summedwith the signal on line 133 from the sign reversing operationalamplifier 144 supplied through potentiometer 145. The output ofamplifier 142 is then fed for sign reversal to amplifier 146 to providean output on line 82.

In FIG. 6 there is shown one circuit for providing the feedforwarddynamics which make up the feedforward controller 70 (FIG. 2) which issimilar to feedforward controller 100 (FIG. 3). The input to controller70 is from line 33 to a circuit branch which includes operationamplifiers 150 and 152 connected serially. The output of 152 on line 154is subjected to integration by integrating amplifier 156 after beingpassed through potentiometer 158 which adjusts the rate time for thefeed-forward control. The result of the integration appears as afeedback to the input amplifier 150 on line 160. The output from line154 is introduced as an input to the final summing amplifier 162 afterpassing through potentiometer 161 which adjusts the rate gain. The otherinput to amplifier 162 is by way of line from line 33. Amplifier 162, ofcourse, produces the output to line 74 after being multiplied by theproportional gain factor K;, as by potentiometer 75 which substitutesfor block 72 of FIG. 2.

In all of the previously mentioned control systems it has been assumedthat the manipulated variable was subject to change without constraint.When, because of the limits in the travel of the valves or otheroperators which modify the manipulated variable there is in effeet aconstraint on changes in the manipulated variable it is necessary tomodify the operation of the control system to take into account thoseconstraints. The most advantageous approach for taking care of suchconstraints is to provide an arrangement for maintaining the input tothe process model during the period of constraint at the value whichexisted when the manipulated variablereached a constraint and thenallowing the process model to change whenever the manipulated variablecan also change. There may thus be used in systems such as that of FIG.2 a circuit in the line 80 for example, which may be referred to as atrack and hold circuit, which will operate so as to allow the input tothe process model 50 to track the signal m, on line 48 as long as thesignal m on line 12 can be followed by the manipulated variable of theprocess 10,

but the circuit then operates to hold the input to the process model 50at the value exists for the signal m, when the manipulated variable ofthe process reaches a constraint, such as a high limit or a low limit.Similar track and hold circuits can be utilized in the systems describedin the other figures so as to maintain the input to the process modelwhich relates to the manipulation of a variable of the process at valvevaluz existing when the manipulated variable reaches a constraint and toallow that input to vary under other conditions.

While the systems disclosed are analog in form it will be evident thatthe steps of the novel control could be carried out by using aprogrammed general purpose digital computer by using well knowntechniques for accomplishing direct digital computer control analogousto the analog control disclosed.

What is claimed is:

I. In a process control system having means for providing both feedbackand feedforward control action to minimize the deviation of a controlledvariable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedbackcontrol action responds so as to cancel those changes of the effectiveerror signal which will be corrected by the anticipated effect of thefeedforward control action.

2. In a process control system having means for providing both feedbackand feedforward control action to minimize the deviation of a controlledvariable from its set point, the improvement whih comprises:

means for modifying the effective error signal to which the feedbackcontrol action responds so that the modification is effective to cancelthat part of the effective error signal which will be corrected by theanticipated effect on the process of the feedforward control action.

3. In a process control system having means for providing both feedbackand feedforward control action to minimize the deviation of a controlledvariable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedbackcontrol action responds so as to prevent a change in the effective errorsignal when the controlled variable changes due to feedforward controlaction.

4. In a process control system having means for providing both feedbackand feedforward control action to minimize the deviation of a controlledvariable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedbackcontrol action responds so as to prevent a change in the effective errorsignal in response to the feedforward portion of that signal to whichthe feedforward control action responds. 5. In a process control systemhaving means for providing both feedback and feedforward control actionto minimize the deviation of a controlled variable from its set point,the improvement which comprises:

means for modifying the effective error signal to which the feedbackcontrol action responds so as to change the effective error signal by anamount substantially equal and opposite to the change in the effectiveerror signal anticipated as a result of the effect on the process of thefeedforward control action.

6. In a process control system having means for providing both feedbackand feedforward control action to minimize the deviation of a controtledvariable from its set point, the improvement which comprises:

means for modifying the effective error signal to which the feedbackcontrol action responds so as to minimize the chance in the effectiveerror signal in response to a change in set point, when that change inset point initiates feedforward control action. 7. In a processcontrol'system having means for providing both feedback and feedforwardcontrol action to minimize the deviation of a controlled variable fromits set point, the improvement which comprises:

means for modifying the effective error signal to which the feedbackcontrol action responds so as to minimize the change inthe effectiveerror signal resulting from a change in the controlled variable due tofeedforward control action from a change in process demand. 8. A processcontrol system for modifying a manipulated variable of the process so asto reduce the deviation of the controlled variable from its set pointcomprising:

feedback control means responsive to the difference between thecontrolled variable and the set point to modify the manipulated variableuntil said controlled variable corresponds with said set point,

feedforward control means responsive to changes in the value of aprocess quantity indicative of a change in the process demand andoperable to modify the manipulated variable as a function of themagnitude of the change of said quantity, said modification being indirection to minimize the deviation of the controlled variable from theset point, and

model means representing said process and operative to respond to saidfeedforward control of said manipulated variable to modify saiddifference between the controlled variable and the set point so as tomodify the effective error signal to which said feedback control meansis responsive to cancel that change in the effective error signal whichwill be corrected by the anticipated effect on the controlled 'variableof the action of the feedforward control means.

9. A process control system as set forth in claim 8 in which thefunction is proportional to the magnitude of the change.

10. A process control system as set forth in claim 8 in which thefunction is the sum of a proportional and a rate response to saidmagnitude of the change in the process quantity.

11. A boiler-turbine control system for modifying the firing rate to theboiler so as to reduce the deviation of the throttle pressure from itsset point comprising:

feedback control means responsive to the difference between the measuredthrottle pressure and its set point to modify the firing rate until saidthrottle pressure corresponds with said set point, feedforward controlmeans responsive to changes in the value of a demand quantity andoperable to modify the boiler firing rate as a function of the value ofsaid demand quantity, said modification being in direction to minimizethe deviation of the controlled variable from the set point, and

model means representing at least a part of said process and operativeto respond to said feedforward control of said firing rate to modifysaid difference between the throttle pressure and its set point so as tomodify the effective error signal to which said feedback control meansis responsive so as to cancel that change in the effective error signalexpected due to the effect on the throttle pressure of the action of thefeedforward control means.

12. A boiler-turbine control system for modifying the firing rate to theboiler so as to reduce the deviation of the throttle pressure from itsset point while changing the set point to maintain the desired outputfrom a generator driven by the turbine comprising:

feedback control means responsive to the difference between the measuredthrottle pressure and its set point to modify the firing rate until saidthrottle pressure corresponds with said set point,

feedforward control means responsive to changes in the value of a demandquantity and operable to modify the boiler firing rate in proportion tothe rate of change of said set point in anticipation of the expectedchange in the value of the measured throttle pressure due to the changein said firing rate, saidmodification being in direction to minimize thedeviation of the controlled variable from the set point, and

model means representing said process and operative to respond to saidfeedforward control of said firing rate to modify said differencebetween the throttle pressure and its set point so as to modify theeffective error signal to which said feedback control means isresponsive so as to cancel that change in the effective error signalexpected due to the effect on the throttle pressure of the action of thefeedforward control means.

13. A process control system comprising:

a feedback controller operable to modify a manipulated variable of saidprocess in response to an effective error signal to reduce saideffective error signal to zero,

a feedforward controller operable tomodify said manipulated variable inresponse to change in a demand signal, and

process model means responsive to the modification of the manipulatedvariable due to action of said feedforward controller for producing asignal for modifying said effective error signal so as to reduce theeffect on said effective error signal of changes in the controlledvariable resulting from action of the feedforward controller in responseto changes in said demand signal.

14. A process control system comprising:

a feedback controller operable to modify a manipulated variable of saidprocess in response to an effective error signal to reduce saideffective error signal to zero,

a feedforward controller operable to modify said manipulated variable inresponse to the change in a set point signal representative of thedesired value for a controller variable,

process model means responsive to the modification of the manipulatedvariable due to action of said feedforward controller for producing afirst signal for modifying said effective error signal so as to reducethe effect on said effective error signal of changes in the controlledvariable resulting from action of the feedforward controller in responseto changes in said set point, and

means for producing a second signal for modifying said effective errorsignal in accordance with said set point signal as modified by a factorrepresentative of the proportional gain of the feedforward controller soas to reduce the effect on said effective error signal of changes insaid set point.

15. The method of controlling a variable of a process to a set pointcomprising the steps of:

dition is the set point.

17. The method of claim 15 in which the process conditionis a demandsignal.

18. The method of controlling a variable of a process to a set pointcomprising the steps of:

proportional to the magnitude of the change in the process condition.

20. The method of claim 18 in which the function is the sum of aproportional and a rate response to said magnitude of the change in theprocess condition.

21. The method of controlling a variable of a process to a set pointcomprising the steps of:

automatically modifying by feedback control action a manipulatedvariable of the process in direction and extent to reduce the effectiveerror between the controlled variable and the set point to zero,

automatically'modifying by feedforward control action the manipulatedvariable of the process in accordance with a function of the change of aprocess condition, and

automatically modifying said effective error signal so as to cancel apredetermined part of the change in the effective error signal inresponse to a change in the controlled variable due to feedforwardcontrol v v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,758,762 a ed September 11, 1973 I Bernard Littman and Charles W.Ross It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

Column l,'-lin e 20, delete "and" .(first occurrence).

Column 9, line 2, after "value" insert -which Column 9, line 8, "valvevaluz" should read --the value--. column 9, line3l, "whih" should readwhic h--.

Column 10, 7, "chance" should read --change-.

"Column 12, line 40, after "prevent" insert --a change--.

' Signed and sealed; this 16th day of July 197i.

(SEAL) Attest:

McCOY M. GIBSON, JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,758,762 Da ed September 11, 1973 Bernard Littman and Charles W. RossIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 20, delete "and" (first occurrence). Column 9, line 2,after "value" insert --which Column 9, line 8, "valve valuz" should read--the value--.

Column 9, line3l, "whih" should read -which--.

Column 10, line 7, "chance" should read -change--.

Column 12, line 40, after "prevent" insert -a change--.

Signedand sealed this 16th day of July 197 (sEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. In a process control system having means for providing both feedbackand feedforward control action to minimize the deviation of a controlledvariable from its set point, the improvement which comprises: means formodifying the effective error signal to which the feedback controlaction responds so as to cancel those changes of the effective errorsignal which will be corrected by the anticipated effect of thefeedforward control action.
 2. In a process control system having meansfor providing both feedback and feedforward control action to minimizethe deviation of a controlled variable from its set point, theimprovement whih comprises: means for modifying the effective errorsignal to which the feedback control action responds so that themodification is effective to cancel that part of the effective errorsignal which will be corrected by the anticipated effect on the processof the feedforward control action.
 3. In a process control system havingmeans for providing both feedback and feedforward control action tominimize the deviation of a controlled variable from its set point, theimprovement which comprises: means for modifying the effective errorsignal to which the feedback control action responds so as to prevent achange in the effective error signal when the controlled variablechanges due to feedforward control action.
 4. In a process controlsystem having means for providing both feedback and feedforward controlaction to minimize the deviation of a controlled variable from its setpoint, the improvement which comprises: means for modifying theeffective error signal to which the feedback control action resPonds soas to prevent a change in the effective error signal in response to thefeedforward portion of that signal to which the feedforward controlaction responds.
 5. In a process control system having means forproviding both feedback and feedforward control action to minimize thedeviation of a controlled variable from its set point, the improvementwhich comprises: means for modifying the effective error signal to whichthe feedback control action responds so as to change the effective errorsignal by an amount substantially equal and opposite to the change inthe effective error signal anticipated as a result of the effect on theprocess of the feedforward control action.
 6. In a process controlsystem having means for providing both feedback and feedforward controlaction to minimize the deviation of a controtled variable from its setpoint, the improvement which comprises: means for modifying theeffective error signal to which the feedback control action responds soas to minimize the chance in the effective error signal in response to achange in set point, when that change in set point initiates feedforwardcontrol action.
 7. In a process control system having means forproviding both feedback and feedforward control action to minimize thedeviation of a controlled variable from its set point, the improvementwhich comprises: means for modifying the effective error signal to whichthe feedback control action responds so as to minimize the change in theeffective error signal resulting from a change in the controlledvariable due to feedforward control action from a change in processdemand.
 8. A process control system for modifying a manipulated variableof the process so as to reduce the deviation of the controlled variablefrom its set point comprising: feedback control means responsive to thedifference between the controlled variable and the set point to modifythe manipulated variable until said controlled variable corresponds withsaid set point, feedforward control means responsive to changes in thevalue of a process quantity indicative of a change in the process demandand operable to modify the manipulated variable as a function of themagnitude of the change of said quantity, said modification being indirection to minimize the deviation of the controlled variable from theset point, and model means representing said process and operative torespond to said feedforward control of said manipulated variable tomodify said difference between the controlled variable and the set pointso as to modify the effective error signal to which said feedbackcontrol means is responsive to cancel that change in the effective errorsignal which will be corrected by the anticipated effect on thecontrolled variable of the action of the feedforward control means.
 9. Aprocess control system as set forth in claim 8 in which the function isproportional to the magnitude of the change.
 10. A process controlsystem as set forth in claim 8 in which the function is the sum of aproportional and a rate response to said magnitude of the change in theprocess quantity.
 11. A boiler-turbine control system for modifying thefiring rate to the boiler so as to reduce the deviation of the throttlepressure from its set point comprising: feedback control meansresponsive to the difference between the measured throttle pressure andits set point to modify the firing rate until said throttle pressurecorresponds with said set point, feedforward control means responsive tochanges in the value of a demand quantity and operable to modify theboiler firing rate as a function of the value of said demand quantity,said modification being in direction to minimize the deviation of thecontrolled variable from the set point, and model means representing atleast a part of said process and operative to respond to saidfeedforward control of said firing rate to modify said differencebetween the throttle pressure and its set point sO as to modify theeffective error signal to which said feedback control means isresponsive so as to cancel that change in the effective error signalexpected due to the effect on the throttle pressure of the action of thefeedforward control means.
 12. A boiler-turbine control system formodifying the firing rate to the boiler so as to reduce the deviation ofthe throttle pressure from its set point while changing the set point tomaintain the desired output from a generator driven by the turbinecomprising: feedback control means responsive to the difference betweenthe measured throttle pressure and its set point to modify the firingrate until said throttle pressure corresponds with said set point,feedforward control means responsive to changes in the value of a demandquantity and operable to modify the boiler firing rate in proportion tothe rate of change of said set point in anticipation of the expectedchange in the value of the measured throttle pressure due to the changein said firing rate, said modification being in direction to minimizethe deviation of the controlled variable from the set point, and modelmeans representing said process and operative to respond to saidfeedforward control of said firing rate to modify said differencebetween the throttle pressure and its set point so as to modify theeffective error signal to which said feedback control means isresponsive so as to cancel that change in the effective error signalexpected due to the effect on the throttle pressure of the action of thefeedforward control means.
 13. A process control system comprising: afeedback controller operable to modify a manipulated variable of saidprocess in response to an effective error signal to reduce saideffective error signal to zero, a feedforward controller operable tomodify said manipulated variable in response to change in a demandsignal, and process model means responsive to the modification of themanipulated variable due to action of said feedforward controller forproducing a signal for modifying said effective error signal so as toreduce the effect on said effective error signal of changes in thecontrolled variable resulting from action of the feedforward controllerin response to changes in said demand signal.
 14. A process controlsystem comprising: a feedback controller operable to modify amanipulated variable of said process in response to an effective errorsignal to reduce said effective error signal to zero, a feedforwardcontroller operable to modify said manipulated variable in response tothe change in a set point signal representative of the desired value fora controller variable, process model means responsive to themodification of the manipulated variable due to action of saidfeedforward controller for producing a first signal for modifying saideffective error signal so as to reduce the effect on said effectiveerror signal of changes in the controlled variable resulting from actionof the feedforward controller in response to changes in said set point,and means for producing a second signal for modifying said effectiveerror signal in accordance with said set point signal as modified by afactor representative of the proportional gain of the feedforwardcontroller so as to reduce the effect on said effective error signal ofchanges in said set point.
 15. The method of controlling a variable of aprocess to a set point comprising the steps of: automatically modifyingby feedback control action a manipulated variable of the process indirection and extent to reduce the effective error between thecontrolled variable and the set point to zero, automatically modifyingby feedforward control action the manipulated variable of the process inaccordance with the magnitude of the change of a process condition, andautomatically modifying said effective error signal so as to prevent achange in the effective error signal due to feedforward control action.16. The method of claim 15 in which the process condition is the setpoint.
 17. The method of claim 15 in which the process condition is ademand signal.
 18. The method of controlling a variable of a process toa set point comprising the steps of: automatically modifying by feedbackcontrol action a manipulated variable of the process in direction andextent to reduce the effective error between the controlled variable andthe set point to zero, automatically modifying by feedforward controlaction the manipulated variable of the process in accordance with afunction of the change of a process condition, and automaticallymodifying said effective error signal so as to prevent in the effectiveerror signal when the controlled variable changes due to feedforwardcontrol action.
 19. The method of claim 18 in which the function isproportional to the magnitude of the change in the process condition.20. The method of claim 18 in which the function is the sum of aproportional and a rate response to said magnitude of the change in theprocess condition.
 21. The method of controlling a variable of a processto a set point comprising the steps of: automatically modifying byfeedback control action a manipulated variable of the process indirection and extent to reduce the effective error between thecontrolled variable and the set point to zero, automatically modifyingby feedforward control action the manipulated variable of the process inaccordance with a function of the change of a process condition, andautomatically modifying said effective error signal so as to cancel apredetermined part of the change in the effective error signal inresponse to a change in the controlled variable due to feedforwardcontrol action.